1. Field of the Invention
The present invention relates to an epitaxial wafer having a gallium nitride (GaN) epitaxial layer deposited on a semiconductor substrate and the method for preparing the epitaxial wafer, which is used for blue and green optoelectronic devices.
2. Description of Related Art
Referring to FIG. 1, a conventional gallium nitride blue light emitting device will be explained. FIG. 1 shows a sectional view of a conventional GaN blue light emitting device. The GaN blue light emitting device comprises an epitaxial wafer including a sapphire substrate 1, a GaN buffer layer 2 deposited on the sapphire substrate 1, and a hexagonal GaN epitaxial layer 3, a first cladding layer 4, a luminescence layer 5, a second cladding layer 6 and a GaN epitaxial layer 7 stacked in the named order on the epitaxial wafer. Ohmic electrodes 8 and 9 are respectively arranged on the GaN epitaxial layers 3 and 7. The GaN buffer layer 2 is disposed to relax distortion caused by difference in lattice parameters between the sapphire substrate 1 and GaN epitaxial layer 3.
Since the conventional light emitting device shown in FIG. 1 uses an insulating sapphire substrate 1, necessary two ohmic electrodes 8 and 9 should be formed at the same side of the substrate 1. This requires at least two times of patterning process by using photolithography. It is also necessary to etch nitride by reactive ion etching, which complicates the manufacturing process of the device. In addition, sapphire has high hardness which make it difficult to divide the devices into individual ones.
In the prior art, it has been tried to use a conductive gallium arsenic (GaAs) substrate instead of the sapphire substrate. For example, Okumura et al. grew cubic GaN on a (100) plane of a GaAs substrate (Journal of Crystal Growth, 164 (1996), pp. 149-153). However, cubic GaN grown on a (100) plane of a GaAs substrate generally has poor quality due to large amount of stacking fault, as shown in a transmission electron microscopy photograph of Okumura et al. This is considered to be caused by instability of cubic GaN which is of higher degree than hexagonal GaN.
On the other hand, it has been also tried to grow more stable hexagonal GaN on a (111) plane of a GaAs substrate. C. H. HONG et al. reported that hexagonal GaN was grown on a (111) A-plane and a (111) B-plane of GaAs substrate by metalorganic vapor phase epitaxy (MOVPE) (Journal of Electronic Materials, Vol. 24, No. 4, 1995, pp. 213-218). However, the grown hexagonal GaN had insufficient properties for use of a blue light emitting device. This is due to low growth temperatures of 800xc2x0 C. at the highest of the GaN epitaxial layer of C. H. HONG et al., contrary to the GaN layer of the blue light emitting device fabricated on a sapphire substrate grown by MOVPE at the growth temperature of higher than 1000xc2x0 C. C. H. HONG et al. grew the GaN epitaxial layer at a low temperature since arsenic having a high vapor pressure would escape from GaAs substrate at a temperature of around 600xc2x0 C.
As mentioned above, in prior art, when hexagonal GaN is epitaxially grown on a GaAs (111) substrate, the substrate temperature is raised to 850xc2x0 C. or so in order to prevent damage of the GaAs substrate caused by heating. By this, the hexagonal GaN epitaxial film obtained by the conventional MOVPE has carrier density of as high as 1xc3x971019 cm31 3 at non-dope, which is not suitable for a blue light emitting device.
Accordingly, it is an object of the present invention to provide a GaN epitaxial wafer having sufficient electronic properties for a blue light emitting device and formed on a GaAs semiconductor substrate and method for preparing the GaN epitaxial wafer, which have overcome the above mentioned defect of the conventional one.
The above and other objects of the present invention are achieved in accordance with the present invention by an epitaxial wafer comprising a (111) substrate of a semiconductor having cubic crystal structure, a first GaN layer having a thickness of 60 nanometers or more, a second GaN layer having a thickness of 0.1 xcexcm or more. The first GaN layer preferably has a thickness of 200 nanometers or less and the second GaN layer has a thickness of 5 xcexcm or less.
According to the invention, the cubic semiconductor (111) substrate is preferably a GaAs (111) substrate. The GaAs (111) A-plane substrate has an advantage that arsenic is less prone to escape and the GaAs (111) B-plane substrate has an advantage that its surface can easily finished by polishing.
According to another aspect of the invention, there is provided a method for preparing an epitaxial wafer comprising the steps of:
supplying a first mixture of material gases comprising a metalorganic including Ga and hydrogen chloride (HCl) and a second mixture of material gases comprising ammonia (NH3) into a chamber heated by a heat source disposed outside of the chamber;
growing a buffer layer by vapor deposition on a substrate at a first temperature disposed in the chamber;
improving crystallinity of the buffer layer by increasing substrate temperature from the first temperature; and
supplying the first and second mixtures of material gases into the chamber in which the substrate is heated at a second temperature higher than the first temperature to grow a GaN layer on the buffer layer.
According to still another aspect of the invention, there is provided a method for preparing an epitaxial wafer comprising the steps of:
supplying a first mixture of material gases comprising HCl and a second mixture of material gases comprising NH3 into a chamber heated by a heat source disposed outside of the chamber;
forming gallium chloride (GaCl) by reacting metal Ga contained in a vessel in the chamber with HCl included in the first mixture of material gases;
growing a buffer layer by vapor deposition on a substrate at a first temperature disposed in the chamber;
improving crystallinity of the buffer layer by increasing substrate temperature from the first temperature; and
supplying the first and second mixtures of material gases into the chamber in which the substrate is heated at a second temperature higher than the first temperature to grow a GaN layer on the buffer layer.
The first substrate temperature is preferably 400-600xc2x0 C. and the second temperature is preferably 850-1000xc2x0 C.
It is preferable that the step of improving crystallinity of the buffer layer by increasing substrate temperature from the first temperature is conducted with supplying NH3 onto the substrate. The GaN layer is preferably grown at a growth rate of 4-10 xcexcm/hour.
The first GaN layer of the epitaxial wafer according to the invention is crystallized by heating a GaN amorphous layer formed at a low temperature of 400-600xc2x0 C. By this, the first GaN layer has a large amount of stacking faults and high impurity densities, such as chlorine, hydrogen, oxygen. This makes the first GaN layer distinguishable from an GaN epitaxial layer formed on it which will be explained hereinafter. The first GaN layer is formed to protect the semiconductor substrate in the succeeding film deposition process at a high temperature. For this purpose, the first GaN layer should be formed at a low temperature of 400-600xc2x0 C., at which the semiconductor substrate is not damaged, and should have a thickness of 60 nanometers or more. According to the invention, a GaAs substrate can be used as a substrate of a GaN epitaxial layer, which is hard to be realized in prior art. It is preferable to supply NH3 onto substrate while the first GaN layer is heated up. This prevents GaN from evaporating and subliming during the heating.
Since the first GaN layer is grown at a low temperature, it has a small growth rate. Therefore, it will spoil manufacturing efficiency if the first GaN layer is formed to have too large thickness. From this point of view, the first GaN layer should have a thickness of not more than 200 nanometers. The effect of protecting the semiconductor substrate will not so improved, if the first GaN layer has a thickness of more than 200 nanometers.
On the other hand, the epitaxial wafer according to the invention comprises a second GaN layer having a thickness of 0.1 xcexcm or more and formed on the first GaN layer. The thickness of the second GaN layer is determined by properties of the light emitting device to be manufactured. However, a thickness of 0.1 xcexcm is too small. The second GaN layer is an epitaxial layer and preferably has a thickness of not more than 5 xcexcm. If the second GaN layer has a thickness of 5 xcexcm, it will be cracked owing to stress. The second GaN layer is preferably grown at a temperature of 850-1000xc2x0 C.
According to the invention, the second GaN layer is preferably grown at a growing rate of 4-10 xcexcm/hour. If its growing rate is lower than 4 xcexcm/hour, the semiconductor substrate may be deteriorated by heat so that the GaN epitaxial layer may peel. In case of the growing rate of higher than 10 xcexcm/hour, the GaN epitaxial layer will have poor crystallinity so that the GaN may have insufficient electronic properties for a blue light emitting device.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.